The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information to anywhere on Earth where there is a line of sight to four or more GPS satellites. GPS provides critical capabilities to military, civil and commercial users around the world. It is maintained by the United States government and is freely accessible to anyone with a GPS receiver.
GPS has non navigation as well as navigation applications. In typical GPS operation as a navigator, four or more satellites must be visible to obtain an accurate result. The solution of the navigation equations gives the position of the receiver along with the difference between the time kept by the receiver's on-board clock and the true time-of-day, thereby eliminating the need for a receiver-based clock.
Applications for GPS such as time transfer, traffic signal timing, and synchronization of various distributed equipment, make use of this accurate timing. An example of a system having distributed units that uses GPS time synchronization is a cell phone system and its cell phone base stations. For time synchronization applications, it is claimed the accuracy of GPS time signals (±10 ns) is second only to the atomic clocks upon which they are based.
GPS is vulnerable to “spoofing”, defined as the transmission of false GPS signals that GPS receivers accept as authentic ones. At its worst, GPS spoofing could allow hackers to disrupt or even to gain control over equipment that rely on GPS for navigation or timing.
Advances are being made into countermeasures to make GPS navigation and timing applications less vulnerable to spoofing and other types of attacks. Of particular threat to timing applications, GPS is vulnerable to timing attacks that are not detectable with current technology.